Opaque White Coating with Non-Conductive Mirror

ABSTRACT

An opaque cover is provided for a capacitive sensor. The cover includes a transparent substrate, and at least one white coating layer including white pigments disposed over at least one portion of the transparent substrate. The cover also includes a non-conductive minor structure disposed over the at least one white coating layer. The non-conductive minor structure includes a number of first dielectric layers having a first refractive index interleaved with second dielectric layers having a second refractive index. The first and second dielectric layers have dielectric constants below a threshold.

TECHNICAL FIELD

Embodiments described herein generally relate to an electronic devicehaving a thin, opaque non-conductive white coating stack. Morespecifically, embodiments relate to an electronic device incorporating athin, opaque white coating stack with a non-conductive mirror layer.

BACKGROUND

Many portable digital devices incorporate at least one display screen toprovide graphical information to a user or viewer. The display screenmay include a liquid crystal display (LCD). Such devices may alsoinclude one or more sensors located beneath a cover glass that overlies,and typically extends beyond, the LCD. As one example, these sensors maybe capacitive sensors.

The devices may also incorporate an opaque region, such as a whitecoating region, outside the display screen (e.g., outside the activedisplay region) but beneath the cover glass. The opaque region mayinclude opaque ink like white ink under a cover glass or sapphire. Thedevices may also incorporate a button, which is one non-limiting andnon-exclusive way to permit a user to provide input to the device. Whenthe button is implemented as or incorporating a mechanical switch, it isoften located within the opaque region. The same may be true when thebutton is a “soft” button, e.g., is a non-moving element that senses atouch and/or force exerted on a surface of the soft button.

A sensor, such as a capacitive fingerprint or touch sensor, may bepositioned under the button. Generally, the white ink should to be thinenough to make the sensor sensitive, but also optically opaque toconceal the sensor and match the coloring of the opaque region.

A white ink having good visual properties may include a high percentageof a pigment such as titanium dioxide (TiO₂), to obtain adequate opticaldensity. However, TiO₂ pigments typically have relatively highdielectric constant, which may affect the operation of a capacitivesensor located beneath the ink layer especially when the white coatingthickness increases. Further, the relative thickness of the ink layermay increase the distance between the sensor and an object it attemptsto sense, such as a finger atop the button. Generally, the sensitivityof a capacitive sensor varies inversely with the square of the distancebetween the sensor and sensed object, so relatively small changes indistance may have large effects on sensor performance. Additionally,particles, voids, and contamination in the black ink or paints mayaffect the performance of the sensor and cause functional errors insensor readings. These issues increase as the thickness of the ink layerused to color the button increases. Therefore, a thinner, non-conductive(or less conductive) white ink may be useful.

SUMMARY

Embodiments described herein may provide a thin opaque non-conductivewhite coating stack that makes a highly sensitive sensor, such as acapacitive sensor, underneath a cover glass or sapphire invisible. Thesensor may provide very clean signal when the cover glass or sapphire istouched. The thin opaque white coating stack may include anon-conductive minor structure that reflects light and may help reducethe thickness of the white coating, such that the sensor may become moresensitive to any touching on the cover glass or sapphire, for example,on a button located on the cover glass outside a display area andgenerates cleaner signal than a thicker white coating. The mirrorstructure also has a relatively low dielectric constant, which helpsimprove the performance of the capacitive sensor. The white coating isvery thin with a thickness ranging from about 20 μm to 25 μm.

In one embodiment, An opaque cover is provided for a capacitive sensor.The cover includes a transparent substrate, and at least one whitecoating layer including white pigments disposed over at least oneportion of the transparent substrate. The cover also includes anon-conductive mirror structure disposed over the at least one whitecoating layer. The non- conductive mirror structure includes a number offirst dielectric layers having a first refractive index interleaved withsecond dielectric layers having a second refractive index. The first andsecond dielectric layers have dielectric constants below a threshold.

In another embodiment, a method is provided for forming a white coatingstack over a substrate. The method includes applying at least one whitecoating layer over at least a portion of a transparent substrate to forma coated substrate. The method also includes forming a non-conductivemirror structure over the coated substrate, wherein the mirror structurehas a dielectric constant lower than a threshold. The method furtherincludes attaching a capacitor sensor to the minor structure.

Additional embodiments and features are set forth in part in thedescription that follows, and in part will become apparent to thoseskilled in the art upon examination of the specification or may belearned by the practice of the embodiments discussed herein. A furtherunderstanding of the nature and advantages of certain embodiments may berealized by reference to the remaining portions of the specification andthe drawings, which forms a part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a perspective view of an electronic device inaccordance with embodiments of the present disclosure.

FIG. 2 illustrates a cross-sectional schematic view of opaque region inaccordance with a first embodiment of the present disclosure.

FIG. 3 illustrates a cross-sectional schematic view of opaque region inaccordance with a second embodiment of the present disclosure.

FIG. 4 illustrates a cross-sectional schematic view of opaque region inaccordance with a third embodiment of the present disclosure.

FIG. 5 illustrates a cross-sectional schematic view of opaque region inaccordance with a fourth embodiment of the present disclosure.

FIG. 6 illustrates a diagram including a capacitive sensor for sensingfingerprint or finger touching in accordance with embodiments of thepresent disclosure.

FIG. 7 is a flow chart illustrating steps for fabricating a displaycover in accordance with embodiments of the present disclosure.

FIG. 8 illustrates a metal block in accordance with embodiments of thepresent disclosure.

FIG. 9 is a flow chart illustrating steps for applying a white ink layerto a substrate or coated substrate in accordance with embodiments of thepresent disclosure.

FIG. 10 shows a stack of a carrier film, a white ink/adhesive layer, anda glass or sapphire substrate in accordance with embodiments of thepresent disclosure.

FIG. 11 is a simplified system diagram for deposition system includingphysical vapor deposition (PVD) in accordance with embodiments of thepresent disclosure.

FIG. 12 illustrates a Scanning Electron Microscope (SEM) image of across-section of the white coating with light absorbing stack includingzirconia oxide (ZrO₂)/tin (Sn).

FIG. 13 illustrates a die coating device in accordance with embodimentsof the present disclosure.

DETAILED DESCRIPTION

The present disclosure may be understood by reference to the followingdetailed description, taken in conjunction with the drawings asdescribed below. It is noted that, for purposes of illustrative clarity,certain elements in various drawings may not be drawn to scale, may berepresented schematically or conceptually, or otherwise may notcorrespond exactly to certain physical configurations of embodiments.

The disclosure provides a minor structure between a white ink or coatinglayer and a highly sensitive sensor, such as a capacitive sensor. Thewhite ink layer may be positioned underneath a glass or sapphire uppersurface, such as a cover glass, and may conceal the capacitive sensorfrom sight. The white ink layer may include white pigments, such astitanium oxide (TiO₂). The minor structure may include a first stack ofsilicon oxide (SiO₂) layers interleaved with niobium pentoxide (Nb₂O₅)layers. Both SiO₂ and Nb₂O₅ have a relatively low dielectric constant ascompared to that of TiO₂. Further, SiO₂ has a different refractive indexfrom that of Nb₂O₅.

The first stack acts like a mirror and reflects light in a broad rangeof visible light. The first stack has a relatively low dielectricconstant and is non-conductive. By reflecting and/or scattering incidentlight through the cover glass, the first stack may help reduce thethickness of the white ink layer, such that the white ink layer may bethin enough to make the capacitive sensor underneath invisible whilestill allowing the capacitive sensor to sense finger touching on thecover glass or sapphire, for example, touching on a button which is apart of the cover glass. A “cover glass,” as used herein, encompassesnot only a transparent covering or layer over an electronic display, butany transparent material overlaying or above a sensor or sensor stack,as incorporated into and placed atop an electronic device. The uppersurface of an input element, such as a mouse, button, switch and thelike, may be an example of a cover glass.

The mirror structure may also include a second stack of silicon oxidelayers interleaved with tin layers. The second stack may act like anisolation layer which further absorbs incident light that may passthrough the first stack or dielectric minor. The second stack includestin as a light absorption element, as tin has relatively high lightabsorption. In some embodiments, tin may be replaced by copper oxide(CuO) or zinc oxide (ZnO) or another light absorption material that isnon-conductive. Silicon oxide is an insulator with a relatively lowdielectric constant, and thus helps improve the performance of thecapacitive sensor as compared to an insulator with a relatively highdielectric constant such as TiO₂.

The mirror structure may be formed by physical vapor deposition (PVD) orother deposition techniques. Methods for applying the white ink layer ona cover glass or sapphire may include heat transfer, among others.

By including the minor structure underneath the white ink layer, thewhite ink layer may be made as thin as 20 μm to 25 μm, which is abouthalf of the conventional 40-50 μm thickness of a white coating loadedwith about 50% TiO₂. A thickness of 20 μm or more may be required toconceal the sensor in the stack and/or to achieve adequate opticaldensity for the opaque cover glass with the mirror structure, forexample, to have at least an optical density of 3 or greater. Athickness of about 25 μm or less may help improve the performance of thecapacitive sensor. Additionally, thinner white coatings may also reducethe amount of TiO₂ and thus the amount of materials in the stack thathave a relatively high dielectric constant. Both a thinner white coatingand reduced amount of high dielectric materials may enhance theeffective range of the capacitive sensor and/or the signal quality.

FIG. 1 illustrates a perspective view of an electronic device accordingto embodiments of the present disclosure. Electronic device 100 mayinclude a display 102 on a surface of a device enclosure (or formingpart of a device enclosure) operative to display information to users.The display 102 may also be a touch sensitive.

The display may be a liquid crystal display (LCD) an organic lightemitting diode (OLED) display, LED display, plasma display, and thelike.

Electronic device 100 may be any of a variety of devices utilizing ahard substrate as a covering or window. The variety of devices mayinclude a mobile phone, tablet computer, notebook computer, instrumentwindow, appliance screen and the like. Electronic device 100 may includea top cover 112, which covers the display 102, and optionally an opaqueregion 104 surrounding the display 102. In the opaque region 104, thecover 112 is partially coated with an opaque coating, such as a whitecoating or a black coating. Cover 112 may have a transparent window(e.g. a glass or sapphire substrate) for viewing the display 102.

As shown in FIG. 1, the opaque region 104 is outside the display 102 orthe active region. Opaque region 104 may include a button 108 as aninput mechanism for controlling the operation of the electronic device.One capacitive sensor or more sensors may be located underneath thebutton 108. An opaque ink layer and/or adhesive may be placed betweenthe bottom surface of the button 108 and the top surface of the sensor.An adhesive may bond the sensor to the button. Opaque region 104 may bewhite or black or any other color.

The sensor may sense a finger touching on the button 108 and generate anelectrical voltage signal. The sensor may also capacitively sense afingerprint through a portion of the cover 112. When the electronicdevice capacitively senses a touch from a user, for example, on thebutton, the device may activate the capacitive sensor at, under or nearthe location at which a touch was sensed. In some embodiments, onlycapacitive sensors corresponding to the touch location may be activatedwhile other capacitive sensors remain inactive.

Top cover 112 is supported by a housing 110. The housing 110 may beformed of a variety of different materials including, but not limitedto, polymer materials (e.g. plastics), metals (e.g. aluminum, steel,etc.), amorphous glass materials, composite materials, and combinationsthereof.

FIG. 2 illustrates a cross-sectional schematic view of opaque region 104in accordance with a first embodiment of the present disclosure. Thecross-section may be taken along arrows A-A in FIG. 1. Stack 200includes a cover substrate 202, such as a cover glass or cover sapphireon the top of the stack, and a white coating layer 204 (which mayinclude white pigment sublayers 204A-D) under the cover glass. Stack 200also includes a gray ink layer 206 under the white coating layer 204.Stack 200 further includes a capacitive sensor 208 at the bottom of thestack. The gray ink layer 206 absorbs light mostly, because the whiteink generally has a high light transmittance while the gray inkgenerally has a low light transmittance. It should be appreciated thatdifferent embodiments may have more or fewer white pigment sublayersthan shown.

The white coating layer may be at least approximately 40˜50 μm thick inorder to provide an optical density of at least 3 or higher, which mayconceal the capacitive sensor 208 underneath the cover substrate 202.With such a large coating thickness, the effectiveness of the sensor 208may be significantly reduced.

FIG. 3 illustrates a cross-sectional schematic view of opaque region 104in accordance with a second embodiment of the present disclosure. Stack300 includes the cover substrate 202 on the top and a white ink layer304 underneath the cover glass. The white ink layer 304 may include anumber of sublayers (not shown). Stack 300 also includes anon-conductive light absorbing stack 306, including one or moredielectric layers (e.g. silicon oxide layers) 306A interleaved withlight absorption layers 306B, such as tin layers 306B. Thenon-conductive light absorbing stack 306 may replace the gray ink layer206 shown in FIG. 2. Stack 300 further includes a capacitive sensor 208attached to the bottom of the non-conductive light absorbing stack 306.

Generally, tin has high light absorption with respect to visiblewavelengths. By using thin tin (Sn) layers 306B interleaved with thickernon-conductive layers/dielectric layers (e.g. SiO₂ layers) 306A, a veryhigh resistance coating with high opacity may be achieved. Tin isnormally conductive. However, tin becomes non-conductive when thethickness of the tin layer is kept under about 100 nm. The tin maynclude some grain structure (or be entirely a grain structure) at suchthicknesses, and thus be non-conductive.

In a particular embodiment, non-conductive light absorbing stack 306 mayinclude seven layers of silicon oxide 306A interleaved with six layersof tin 306B, or more generally N layers of silicon oxide interleavedwith N−1 tin layers. As shown in FIG. 3, a top silicon oxide layer 306Ais attached or adjacent to the white ink layer 304, while a bottomsilicon oxide layer 306A is attached, adjacent or near the capacitivesensor 208. In some embodiments, one silicon oxide layer may have adifferent thickness from another silicon oxide layer. Likewise, one tinlayer may have a different thickness from another tin layer. In aparticular embodiment, each of the silicon oxide layers or each of thetin layers has substantially the same thickness. The white ink layer 304may include white ink or pigment sublayers 204A-D and may have athickness of about 40-50 μm to make the capacitive sensor invisible fromthe cover substrate 202 on the top.

In some embodiments, non-conductive light absorbing stack 306 mayinclude seven zirconia oxide (ZrO₂) layers 306A interleaved with six tinlayers 306B. One of the benefit of the ZrO2 is that it has very goodoxygen permeability. The reason for the very good oxygen permeability isdue to the high porous microstructure. FIG. 12 illustrates a ScanningElectron Microscope (SEM) image of a cross-section of the white coatingwith light absorbing stack including zirconia oxide (ZrO₂)/tin (Sn). TheZrO₂/Sn 1204 shows a porous structure above white ink 1202. The goodoxygen permeability helps minimize discoloration due to UV light duringthe use of the electronic device.

In a particular embodiment, the tin layers may be about 15 nm thick sothat the tin layer is non-conductive. The top ZrO₂ layer and the bottomZrO₂ layer may be thicker than the middle ZiO₂ layers in the lightabsorbing stack 306 for better oxygen permeability. The top and bottomZrO₂ layers may be 100 nm thick, while the middle ZrO₂ layers may beabout 30 nm thick such that the total thickness of the light absorbingstack 306 may be about 1070 nm thick. It will be appreciated by thoseskilled in the art that the thicknesses of these layers may vary.

In the present disclosure, the gray ink layer 206 or the non-conductivelight absorbing stack 306 may be replaced with a non-conductive mirrorstructure, which may be fabricated by a deposition method, such asphysical vapor deposition, chemical vapor deposition, ion beamdeposition, or sputter deposition among others. The mirror structure mayhelp reduce the thickness of the white coating or white coating layer tonearly half its normal thickness, i.e. from about 40 μm˜50 μm to about20 μm˜25 μm. The reduction in thickness of the white coating is achievedby scattering and/or reflecting light by the minor structure such thatthe optical density is maintained as the same level as the thicker whitecoating without the minor structure, such as the same as for stacks 200or 300. As a result, the white coating may be made thinner, which mayreduce the distance between the sensor and a sensed object.

FIG. 4 illustrates a cross-sectional schematic view of region 104 inaccordance with a third embodiment of the present disclosure. Stack 400includes the cover substrate 202 on the top of the stack, a white inklayer 404 underneath the cover glass, and a capacitive sensor 208 at thebottom of the stack. The white ink layer 404 may include sublayers 404Aand 404B in a particular embodiment.

Stack 400 also includes a non-conductive mirror structure 406 positionedbetween the white ink layer 404 and the capacitive sensor 208. The minorstructure 406 includes a light reflection stack 406A that reflectsand/or scatters incident light 210 back to the white ink layer 404. Thereflection stack 406A may be formed from multiple layers of alternatingdielectric materials, at least some of which may have differentrefractive indexes. For example, a first and second dielectric material,each with a different refractive index, may be interleaved with oneanother to form the alternating layers. The mirror structure 406 mayalso include a non-conductive light absorbing stack 406B under the lightreflection stack 406A. The non-conductive light absorbing stack 406B maybe similar to non-conductive light absorbing stack 306, and may absorblight passing through the light reflection stack 406A. That is, thelight reflection stack 406A may not completely reflect all incidentlight and so a portion of the incident light may imping on the lightabsorbing stack 406B.

The mirror structure 406 may replace the gray ink layer 206, shown inFIG. 2, or the non-conductive light absorbing stack 306 of silicon oxidelayers 306A interleaved with tin layers 306B, shown in FIG. 3.

The thinned white coating with the minor structure generally has lessTiO₂ than a typical TiO₂-based white coating with equivalent opticaldensity to the thinned white coating, and thus has a lower effectivedielectric constant in comparison. The thinned white coating and lowereffective dielectric constant may help the sensor to provide a muchcleaner signal, i.e. a signal with significantly improved ratio ofsignal-to-noise, and/or sense objects at a greater distance.

FIG. 5 illustrates a cross-sectional schematic view of an opaque region104 in accordance with a fourth embodiment of the present disclosure.Stack 500 may include a cover substrate 202 and a white ink layer 404formed of white coating sublayers underneath the cover glass. Stack 500also includes a capacitive sensor 208 at, or adjacent, the base of thestack. Stack 500 further includes a mirror structure 406 between thesensor 208 and the white ink layer 404.

The mirror structure 406 may include a light reflection stack 406Aformed from interleaved first and second dielectric layers. The firstdielectric layers may have a first refractive index 506A while seconddielectric layers may have a second, different refractive index 506B.Both first and second dielectric layers may have a relatively lowdielectric constant. For example, the first dielectric layer may beformed from SiO₂ which has a relatively low dielectric constant (e.g.,about 3.9). By contrast, the dielectric constant of a typical TiO₂-basedwhite pigment is 86-173. SiO₂ further is a light diffuser, having arefractive index of about 1.5. SiO₂ is also a common electricalinsulator.

The second dielectric layer may be formed from niobium oxide (Nb₂O₅),which hasa relatively low dielectric constant of about 42. Nb₂O₅ is alsoan electrical insulator.

Additionally, the first and/or second dielectric layers may be formedfrom Si₃N₄ which has a relatively low dielectric constant of about 7.5,and a refractive index of about 2 (e.g., between the refractive indexesof SiO₂ and Nb₂O₅). Each of SiO₂, Si₃N₄, and Nb₂O₅ has lower orsignificantly lower dielectric constant than the dielectric constant 3of titanium oxide (TiO₂).

The mirror structure 406 may vary in reflectivity between embodiments,for example depending on the difference between the refractive index oftwo alternating dielectric layers. The thickness of the dielectriclayers may affect the wavelength at which light may be reflected.

It may be useful to use an inorganic dielectric, such as an oxide ornitride, with a relatively low dielectric constant. In some embodiments,a thin polymer film may be used. The oxides or nitrides may be depositedby vacuum technology to form very thin films.

The light reflection stack 406A formed of alternating dielectric layershaving different refractive indexes generally functions as anon-conductive mirror, based on the interference of light reflected fromthe alternating dielectric layers. In a particular embodiment, the lightreflection stack 406A may include thin layers having a relatively highrefractive index interleaved with thicker layers having a relatively lowrefractive index.

The mirror structure 406 may also include a non-conductive lightabsorbing stack 406B located underneath the light reflection stack 406A.The non-conductive light absorbing stack 406B, 306 may include a numberof dielectric layers 306A, such as silicon oxide layers, interleavedwith light absorbing layers 306 B, such as tin (Sn) layers. Thisstructure is shown generally in FIG. 3. As previously mentioned, thenon-conductive light absorbing stack 406B, 306 may absorb at least somelight that passes through the light reflection stack 406A.

The thickness of the tin in the non-conductive light absorbing stack406B, 306 may be kept under about 100 nm in order to ensure the tinlayer is non-conductive. The tin layer is typically a grain structurerather than a continuous structure. The non-conductive light absorbingstack 406B, 306 may simulate a gray ink layer 206 in certainembodiments.

Additionally, tin has an electrical resistivity greater than 10⁶ Ωcmwhen the thickness of tin is less than 100 nm(for example, where a 40 nmthick tin layer is used) which is still much lower than that of SiO₂.Thus, the inclusion SiO₂ may increase the electrical resistivity of thenon-conductive light absorbing stack 406B.

In alternative embodiments, other materials may replace tin in the lightabsorption layers. For example, copper oxide (CuO) generally has goodlight absorption qualities and may form a non-conductive layer, or beused as part of a non-conductive layer. Zinc oxide (ZnO) may also beused as a light absorption layer and likewise has good resistivity.

In alternative embodiments, SiO₂ in the layers may be replaced bysilicon nitride, such as Si₃N₄, or other oxides. Materials used in thelayers described herein may vary in electrical resistivity. For example,Si₃N₄ generally has an electrical resistivity of 10¹⁴ Ωcm, which islower than the electrical resistivity of 10¹⁶ Ωcm for SiO₂.

Each of layers 506A-B and 306A-B in the mirror structure 406 may vary inthickness. In some embodiment, one SiO₂ layer or Nb₂O₅ layer may have adifferent thickness from another SiO₂ layer or Nb₂O₅ layer. In a sampleembodiment, each of the SiO₂ layers and/or each of Nb₂O₅ layers hassubstantially the same thickness. The layer thickness(es) of the lightreflection stack 406A and/or the non-conductive light absorbing stack406B may be selected to achieve targeted optical and electricalproperties, such as a certain light reflectivity, wavelength range,light absorption, electrical resistivity and so on. Such properties mayalso be controlled by varying the thicknesses or materials or number oflayers, such as the SiO₂ layer 506A, Nb₂O₅ layer 506B, tin layer 306Aand/or SiO₂ layer 306B. In some embodiments, stack 500 may achieve anoptical density of at least 3 and/or a sufficiently high electricalresistance that the stack 500 is essentially non-conductive.

It will be appreciated by those skilled in the art that other opticalstackups may be used for the minor structure 406. For example, themirror structure 406 may include a non-conductive vacuum metallization(NCVM) film.

The cover substrate 202 may be optically transparent and may be formedfrom a variety of materials, such as glass, chemically strengthenedglass, sapphire, plastic and so on. Generally, sapphire may beanisotropic and may facilitate operation of the capacitive sensor.

In various embodiments, the button and/or the cover substrate may beflat, curved, circular, square, and/or rectangular. It will beappreciated by those skilled in the art that the button and/or coversubstrate may vary in shape and/or dimension.

The operation of the capacitive sensor will now be briefly discussed.The capacitive sensor detects a change in capacitance when a user'sappendage (or a suitable object, such as a stylus) approaches or touchesthe sensor. There is a fringe electric field that extends from thecapacitive sensor 208 beyond the cover substrate 202. The electricalenvironment changes when the appendage enters the fringe field, with aportion of the electric field being shunted to ground instead ofterminating at the capacitive sensor. As a result, the capacitance ofthe capacitive sensor 208 decreases, which can be detected.

FIG. 6 illustrates a diagram of a sample a capacitive sensor for sensingfingerprints and/or touch (or near-touch) in accordance with embodimentsof the present disclosure. It should be appreciated that the capacitivesensor is meant as an example only; other sensors (whether capacitive ornot) may be used in various embodiments. For example, optical,pyroelectric, capacitive swipe, ultrasonic and other sensors may be usedin different embodiments, and so the discussion the capacitive sensorset forth with respect to FIG. 6 is meant to be an example only.

The capacitive sensor 208 may be used to provide secure access tosensitive electronic devices and/or data. As shown in FIG. 6, thecapacitive sensor 208 may include both an array of capacitive sensingelements 602 and drive ring 604. The capacitive sensing element 602 mayinclude data or other information with respect to a relatively smallregion of a fingerprint image. Generally, the capacitive sensor 208 maybe used to determine an image of a fingerprint through measuringcapacitance through each capacitive sensing element 602 of thecapacitive sensor 208.

The voltage of the array of capacitive sensing elements 602 is notdirectly driven or modulated, but instead drive ring 604 is modulated bya drive amplifier 606. This modulation, in turn, excites finger 608 andthe voltage and/or charge at each capacitive sensing element 602 variesas drive ring 604 is modulated since finger's 608 voltage changes withthe modulation of drive ring 604.

For the capacitive sensor, the voltage applied to the drive ring 604 maybe limited. Generally, the voltage is no more than a threshold of 4volts (peak-to-peak). Any voltages above this threshold for exciting thefinger 608 may be detected by a person as a “tingling” or uncomfortablefeeling in his or her finger. Although the exact voltage at which onecan sense the tingling may vary from person to person, the 4 voltpeak-to-peak voltage is generally considered as the threshold beyondwhich the uncomfortable feeling is noticeable.

Since the voltage of the drive ring may be restricted to avoid userperception, the thickness of any dielectric overlaying the sensor islimited. Generally, the capacitance between the sensor 208 and finger608 decreases with increased spacing between the sensor and finger orthe thickness of the dielectric layer or stack between the sensor andfinger. For example, when the finger is away from the sensor 208, alower capacitance may be generated between the sensor and finger, andthus lower voltage signals are produced on underlying capacitive sensingelements 602. By contrast, when the finger is closer to the sensor 208,a higher capacitance may be generated between the sensor and finger, andthus higher voltage signals are produced on underlying capacitivesensing elements. With reduced capacitance, the fingerprint image maybecome blurry. As discussed above, by reducing the white coatingthickness and employing the minor structure 406, the performance of thesensor may be improved.

FIG. 7 is a flow chart illustrating steps for fabricating a displaycover including a white coating and a sensor under the white coating inaccordance with embodiments of the present disclosure. Method 700 startswith operation 702, in which a white coating is applied on at least oneportion of a transparent cover substrate 202. The white coating or layermay be applied to a button 104 or other region outside the display area,for example.

Method 700 continues in operation 706 with the operation of formingminor structure 406 over the coated substrate. The coating method mayinclude, but is not limited to, physical vapor deposition (PVD),chemical vapor deposition (CVD), plasma enhanced chemical vapordeposition (PECVD), and/or ion beam assisted deposition (IBAD), amongothers. Method 700 further may include the operation of attaching acapacitive sensor 208 to an opposite side of the minor structure 406from the transparent cover substrate, as shown in operation 710.

Various techniques for applying a white coating and forming a mirrorstructure on a cover glass or sapphire substrate are discussed below.

Processes for Applying White Coating on Glass Substrate

The white coating 204, 304, or 404 may be applied to a glass substratein accordance with various methods.

In one embodiment, a silk screen is used. The silk screen includes awoven mesh that transfers ink or printable materials onto a substrate. Afill blade or squeegee is moved across the silk screen, forcing the inkinto the openings of the woven mesh to transfer by capillary actionduring a squeegee stroke. The silk screen method may have issues withcoating thickness uniformity due to the mesh. For example, it maygenerate about a 1 μm height difference which may affect the performanceof the capacitor sensor 208.

In another embodiment, a slit coating process may be used. Slit coatingis a process that creates an uninterrupted curtain of fluid that fallsonto a substrate. The substrate is transported on a conveyor belt at aconstant speed through the curtain to ensure an even coat on thesubstrate. The curtain is created by using a slit at the base of aholding tank, such as a metal block, thereby allowing the liquid to fallupon the substrate.

FIG. 8 illustrates a metal block suitable for use in slit coating and inaccordance with embodiments of the present disclosure. As shown, a metalblock 802 may include a reservoir material 804, such as a white ink orwhite pigments in liquid form. The metal block 802 also includes a slit806 at the bottom. The slit 806 allows white ink 804 to pass through toform a coating on a moving substrate.

In another embodiment, a heat transfer method may be used to deposit thewhite ink. Specifically, the heat transfer method uses a carrier film toroll white ink onto the carrier film, and applies the white ink to aglass or sapphire substrate by heating, followed by peeling off thecarrier film from the glass or sapphire substrate.

FIG. 9 is a flow chart illustrating steps for applying a white ink layerto a substrate or coated substrate in accordance with embodiments of thepresent disclosure. Method 900 may start with rolling a white inksublayer onto a carrier film, which may be a flexible polymer film, suchas a polyethylene(terephthalate) (PET) film, at operation 902. Operation906 includes attaching the carrier film with the white ink layer to aglass or sapphire substrate 202 heating and applying pressure to thecarrier film while against the substrate 202, such that the white inklayer adheres to the substrate 202. The white ink or pigments may beembedded within adhesives in certain embodiments.

Method 900 may continue with cooling the heated substrate with thecarrier film to form a coated substrate at operation 910, followed bypeeling off the carrier film from the coated glass substrate atoperation 914.

FIG. 10 shows a stack of a carrier film 1002, a first white ink/adhesivesublayer 404A, and a cover substrate 202 in accordance with embodimentsof the present disclosure. An additional white coating sublayer 404B maybe applied to the coated cover substrate, along with white coatingsublayer 404A, by repeating the method disclosed with respect to FIG. 9.This may provide a substantially homogeneous and uniform opaque thinfilm.

In a particular embodiment, each of the white coating sublayers 404A-Band 204A-D may be about 10 μm thick. Thus, heat transfer operations mayprovide a white coating 404 of about 20 μm to 25 μm, including twosublayers of white coating 404A, 404B. Such a thin white coating 404 mayhave an optical density of at least 3 or greater and may also haveminimal impact on the performance of the sensor 208. Similarly, the heattransfer method may provide a white coating 204 of about 40 μm to 50 μm,including four sublayers of white coating 204A-D.

FIG. 13 shows a die coating device in accordance with embodiments of thepresent disclosure. The die coating device 1300 includes a slit die1302, a transfer roller 1304, and a carrier film 1308, such as a PETfilm. The slit die has a slit opening 1306 toward the transfer roller1304 such that when the transfer roller rotates, the ink transfers tothe carrier film 1308. As shown, the white ink is pressed into an inputopening 1310 of the slit die 1302 by a ink flow pressure and is outputfrom the slit opening 1306 to transfer to the carrier film 1308 on thetransfer roller 1304. The carrier film 1308 is then coated with auniform layer of white ink while the transfer roller 1304 may rotate ata constant feed rate.

The coating thickness may be controlled by several key factors,including distance of the transfer roller 1304 to the slit opening 1306,the ink flow pressure, the feed rate of the transfer roller 1304, andink viscosity, among others. The coating thickness formed may be thin,for example, may be equal to or less than 20 μm. One of the benefits ofthe die coating process is that the white coating may have very uniformthickness.

In still another embodiment, spin coating may be used to deposit thewhite ink. Spin coating is a procedure that is used to deposit uniformthin films on flat substrates. Generally, a small amount of coatingmaterial is applied on the center of a substrate. The substrate is thenrotated at high speed in order to spread the coating material bycentrifugal force. This spin coating method may create thin films withthicknesses below 10 nm. Therefore, more white coating sublayers may bedeposited to form a white coating, as disclosed in FIGS. 2-5.

Process for Fabricating Minor structure

The various layers of the minor structure 406 discussed herein may beformed or deposited over the substrate 202 in a variety of manners. Forexample, deposition technologies may include PVD, CVD, PECVD, and/orIBAD, each of which may produce layers having slightly differentstructures. These different structures may affect electrical propertiesand/or optical properties of the minor structure, among others. Thedeposition of coating materials varies by process, with the specificconditions—including the atmosphere, the temperature of the substrateand chamber, the pressure, presence, ratio, type and energy ofadditional energetic ions, the deposition rate and the condition of thecoating materials—all contributing to the final structure, compositionand density that can affect the various material properties.

FIG. 11 is a simplified system diagram for a PVD system according toembodiments of the present disclosure. A deposition system 1100 mayapply a surface treatment to the substrate 202. In this particularexample, deposition system 1100 includes one or more reservoirs 1110holding various coating materials 1108 (e.g., SiO₂, Si₃N₄, Sn, andNb₂O₅). An inert gas 1112 (e.g., argon or nitrogen) may be supplied bygas source 1116 through a purge or pressurization flow pipe 1114 inorder to reduce oxidation, wetting andlor contamination withinreservoirs 1110.

Depending on the design, reservoirs 1110 may be coupled to a vacuumchamber 1118 by one or more delivery tubes 1122, which may be configuredto deliver materials 1108 from reservoirs 11110 to supply systems 1120.Supply systems 1120 typically utilize a suitable combination of tubes,pumps, valves and other components to direct materials 1108 intovaporizing or deposition units 1126 for deposition onto substrate 202,as shown in FIG. 2. In the particular configuration of FIG, 11,deposition units 1126 are provided in the form CVD or PVD components.Alternatively, other processes and components may be utilized to treatthe substrate 202, examples of which include sputtering, electron beamdeposition or electron beam evaporation, WAD, PECVD and/or a combinationof such processes.

In some embodiments, deposition system 1100 also controls pressure,temperature and humidity to operate chamber 1118 as a vacuum chamber orother chemical or physical vapor deposition environment. Depositionsystem 1100 may also maintain a particular temperature for the surfacecoating process, for example, between about 100° C. and about 150° C.,or between about 100° C. and about 170° C. Air may also be providedwithin chamber 1118, either during or after the coating process, inorder to expose substrate 202 to atmosphere in a controlled process,before removal from chamber 1118.

In general, supply systems 1120 and deposition units 1126 are controlledto deposit selected amounts of material (e.g.., SiO₂, Sn, and Nb₂O₅)onto substrate 202 in particular orders and combinations.

Referring to FIG. 1 again, the opaque region 104 (except button 108, oranother region of the cover glass overlying a capacitive sensor) may usea conventional printing method to form a relatively thicker whitecoating. Under the button 108 (or other region), the mirror structuremay be used to help reduce the white coating thickness to about half ofthat of the opaque region 104. In some embodiments, the mirror structuremay be used under the entire opaque region 104.

Having described several embodiments, it will be recognized by thoseskilled in the art that various modifications, alternativeconstructions, and equivalents may be used without departing from thespirit of the disclosure. Additionally, a number of well-known processesand elements have not been described in order to avoid unnecessarilyobscuring the embodiments disclosed herein. Accordingly, the abovedescription should not be taken as limiting the scope of the document.

Those skilled in the art will appreciate that the presently disclosedembodiments teach by way of example and not by limitation. Therefore,the matter contained in the above description or shown in theaccompanying drawings should be interpreted as illustrative and not in alimiting sense. The following claims are intended to cover all genericand specific features described herein, as well as all statements of thescope of the present method and system, which, as a matter of language,might be said to fall therebetween.

1. An opaque cover for a capacitive sensor, the cover comprising: atransparent substrate; at least one white coating layer including whitepigments disposed over at least one portion of the transparentsubstrate; and a non-conductive mirror structure disposed over the atleast one white coating layer, wherein the non-conductive mirrorstructure comprises a first plurality of first dielectric layers havinga first refractive index interleaved with second dielectric layershaving a second refractive index, wherein the first and seconddielectric layers have dielectric constants below a threshold.
 2. Theopaque cover of claim 1, wherein the non-conductive mirror structurecomprises a second plurality of light absorption layers interleaved withthird dielectric layers, the third dielectric layers having a dielectricconstant below the threshold.
 3. The opaque cover of claim 2, whereinthe light absorption layer comprises a material selected from a groupconsisting of tin, copper oxide, and zinc oxide.
 4. The opaque cover ofclaim 3, wherein the tin layer has a thickness equal to or less than 100nm such that the tin layer is non-conductive.
 5. The opaque cover ofclaim 2, wherein the third dielectric layers comprises a materialselected from a group consisting of silicon oxide, silicon nitride, andniobium oxide.
 6. The opaque cover of claim 1, wherein each of the firstand second dielectric layers comprises a material selected from a groupconsisting of silicon oxide, silicon nitride, and niobium oxide.
 7. Theopaque cover of claim 1, wherein the first dielectric layers comprisesilicon oxide, and the second dielectric layers comprises niobium oxide.8. The opaque cover of claim 1, wherein the non-conductive mirrorstructure comprises a first stack of thirteen silicon oxide layersinterleaved with thirteen niobium oxide layers.
 9. The opaque cover ofclaim 8, wherein the first stack of thirteen silicon oxide layersinterleaved with thirteen niobium oxide layers has a total thickness ofabout 2.0 μm.
 10. The opaque cover of claim 8, wherein thenon-conductive mirror structure comprises a second stack of sevensilicon oxide layers interleaved with six tin layers.
 11. The opaquecover of claim 10, wherein the second stack of seven silicon oxidelayers interleaved with six tin layers has a total thickness of about1.2 μm.
 12. The opaque cover of claim 8, wherein the non-conductivemirror structure comprises a second stack of seven zirconia oxide layersinterleaved with six tin layers.
 13. The opaque cover of claim 10,wherein the second stack of seven zirconia oxide layers interleaved withsix tin layers has a total thickness of about 1.1 μm.
 14. The opaquecover of claim 1, wherein the transparent substrate comprises sapphireor glass.
 15. The opaque cover of claim 1, wherein the white coatinglayer comprises titanium oxide.
 16. The opaque cover of claim 1, whereinthe white coating layer has a thickness ranging from 20 μm to 25 μm. 17.The opaque cover of claim 1, wherein the opaque cover has an opticaldensity of at least 3 or greater.
 18. The opaque cover of claim 1,wherein the sensor is configured to attach to the mirror structure suchthat the sensor is invisible through the transparent substrate.
 19. Amethod for forming a white coating stack over a substrate, the methodcomprising: applying at least one white coating layer over at least aportion of a transparent substrate to form a coated substrate; forming anon-conductive mirror structure over the coated substrate, wherein themirror structure has a dielectric constant lower than a threshold; andattaching a capacitor sensor to the mirror structure.
 20. The method ofclaim 17, wherein the at least one white coating layer has a thicknessranging from 20 μm to 25 μm.
 21. The method of claim 17, whereinapplying at least one white coating layer over at least a portion of asubstrate comprises: rolling a layer of white pigments onto a carrierfilm, the white pigments being embedded within adhesives; attaching thelayer of white pigments onto a substrate by heating the substrate andapplying pressure to the carrier film against the substrate; cooling theheated substrate with the carrier film to form a coated substrate; andpeeling off the carrier film from the coated substrate.
 22. The methodof claim 17, wherein forming a non-conductive mirror structure over thecoated substrate is a method selected from a group consisting ofphysical vapor deposition (PVD), chemical vapor deposition (CVD), plasmaenhanced chemical vapor deposition (PECVD), and ion beam assisteddeposition (IBAD).
 23. The method of claim 17, wherein thenon-conductive mirror structure comprises a first plurality of firstdielectric layers having a first refractive index interleaved withsecond dielectric layers having a second refractive index, wherein thefirst and second dielectric layers have dielectric constants below athreshold.
 24. The method of claim 21, wherein each of the first andsecond dielectric layers comprises a material selected from a groupconsisting of silicon oxide, silicon nitride, and niobium oxide.
 25. Themethod of claim 21, wherein the non-conductive mirror structurecomprises a second plurality of light absorption layers interleaved withthird dielectric layers, the third dielectric layers having a dielectricconstant below the threshold.
 26. The method of claim 23, wherein thelight absorption layer comprises a material selected from a groupconsisting of tin, copper oxide, and zinc oxide.
 27. The method of claim24, wherein the tin layer has a thickness equal to or less than 100 nmsuch that the tin layer is non-conductive.
 28. The method of claim 23,wherein the third dielectric layers comprises a material selected from agroup consisting of silicon oxide, silicon nitride, and niobium oxide.29. The method of claim 17, wherein the non-conductive mirror structurecomprises a first stack of thirteen silicon oxide layers interleavedwith thirteen niobium oxide layers.
 30. The method of claim 21, whereinthe first stack of thirteen silicon oxide layers interleaved withthirteen niobium oxide layers has a total thickness of about 2.0 μm. 31.The method of claim 21, wherein the non-conductive mirror structurecomprises a second stack of seven silicon oxide layers interleaved withsix tin layers.
 32. The method of claim 29, wherein the second stack ofseven silicon oxide layers interleaved with six tin layers has a totalthickness of about 1.2 μm.
 33. The method of claim 17, wherein thetransparent substrate comprises sapphire or glass.
 34. The method ofclaim 17, wherein the white coating stack has an optical density of atleast 3 or greater.